The detection of the concentration of glucose in urine is of great importance to diabetic individuals who must control their blood sugar level. Because continued control of blood sugar level is so important, a test device for the determination of glucose concentration in urine must be rapid and simple enough for the individual to use with ease and it must be quantitative enough to accurately measure increments of glucose concentration and thereby differentiate between various medical conditions.
A novel and very effective device for the quick and convenient determination of urine glucose concentration is disclosed in U.S. Pat. No. 3,814,668 (the '668 patent), issued June 4, 1974. This patent discloses a test strip which comprises a carrier member impregnated with glucose oxidase, a peroxidative active substance and a water soluble iodide salt which is coated with a hydrophobic, semi-permeable membrane such as ethyl cellulose.
The semi-permeable membrane coating is designed to prevent the retention of excess liquid sample on the test device and therefore provide more reliable semi-quantititive results. The patent teaches that a reasonably wide range of concentrations of materials can be used and more specifically discloses the application of the semi-permeable membrane from a solution containing 0.5 to 1 g of film forming polymer, e.g., ethyl cellulose, per 100 ml of organic solvent.
Test strips made by the process in the '668 patent are highly effective for the intended purpose. However, a phenomenon known as "windowframing" or "edgewetting" has been observed with these strips. Windowframing may reveal itself through the development of very small isolated wet spots around the margin of the strip after dipping. In some cases, a continuous wet margin around the reacted strip may be observed. The windowframing problem is strictly a phenomenon occuring at the edge of the strips, since the strips are prepared by cutting a sheet of the bibulous carrier after it is coated. Since the wet spots or wet margin are invariably of a different color from the rest of the reagent strip, visual matching of the reacted strips to color charts becomes difficult and may possibly lead to incorrect results. One solution to the problem has been to carefully control the ethoxyl content and viscosity of the ethyl cellulose. This technique introduces an additional parameter which increases quality control and raw material procurement problems.
It would be desirable, and it is an object of this invention, to provide a novel process for the preparation of the previously described glucose test strips which eliminates the problem of windowframing.
An additional object is to provide such a process which eliminates the problem of windowframing without slowing down the test strip's reactivity to an unacceptable rate.
A further object is to provide a process which permits the use of a wider range of ethyl cellulose formulations than was possible using prior art coating techniques.
An additional object is to provide an improved test strip prepared by the process described herein.